1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device. More specifically, the invention relates to a one-time programmable (OTP) memory using irreversible storage elements to which information can be written only once.
2. Description of the Related Art
Recently, an OTP memory using storage elements of an insulating film breakdown type, such as an anti-fuse element (or an electrical-fuse element), has been proposed as an irreversible storage element. The OTP memory stores information by applying a high voltage that exceeds the maximum rating to a semiconductor element having a metal oxide semiconductor (MOS) structure and electrically breaking a gate insulating film of the semiconductor element. The OTP memory using the anti-fuse element as a storage element is used chiefly to store information “0” in the anti-fuse element whose insulating film has not been broken down and information “1” in the anti-fuse element whose insulating film has been broken down.
The OTP memory is also used to store information for replacing a defective element with a dynamic random access memory (DRAM) or the like.
In order to read information stably from the anti-fuse element, the broken gate insulating film needs to be brought into good electrical conduction. It is thus important to apply adequate electrical energy (current stress) to the broken gate insulating film to break it down completely or bring it into a hard breakdown.
As described above, a considerably large amount of current needs to flow through a gate insulating film for a given period of time when the OTP memory is programmed (information is written to the OTP memory). If, however, the current continues to flow for a longer time than required, an excessive stress is applied to circuits other than the anti-fuse element including the gate insulating film. The read characteristic therefore deteriorates conversely. For example, there occurs a problem that read current is reduced by hollows caused in wires by migration due to a large amount of current.
In general, times required for breaking down a gate insulating film, or program times required from when a high voltage is applied to an anti-fuse element until information is written to the anti-fuse element vary among anti-fuse elements. Even though the OTP memory is programmed by applying such a high voltage as to break down the gate insulating films of a number of anti-fuse elements instantaneously (not longer than 10 μs) for a given period of time (about 100 μs), anti-fuse elements whose gate insulating films are not broken down completely, or anti-fuse elements that fail to store information remain with a certain probability. The program times are sensitive to variations in environmental conditions such as applied voltage and temperature. These problems should be resolved in order to appropriately control electrical energy applied to an anti-fuse element whose gate insulating film has been broken down.
In order to simply lower the incidence of anti-fuse elements that fail to store information, it is effective to apply a high voltage, apply a voltage for a long time, or the like. If, however, these measures are taken, it is likely that a greater electrical stress will be applied to a number of anti-fuse elements whose gate insulating films are broken down at early time. To increase a voltage to be applied becomes a great stress on anti-fuse elements which are not programmed or to which no information is written. It is not favorable in terms of reliability. To lengthen the time of application of voltage increases the program time and prevents the OTP memory from increasing in storage capacity.
In the above prior art OTP memory, the electrical energy to be applied to the broken gate insulating film of an anti-fuse element needs to be controlled appropriately in order to break down the gate insulating film completely (hard breakdown).
A method of controlling electrical energy applied to a gate insulating film has been proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-123496 (corresponding to U.S. Pat. No. 6,434,060 B1). This method has the problems that the electrical energy applied to the broken gate insulating film cannot be controlled appropriately and the device increases in size as the storage capacity becomes large.